Fluid energy drying and grinding mill

ABSTRACT

A fluid energy mill of the confined vortex type is designed to permit the simultaneous drying and grinding of slurries of pulverulent solids, e.g., pigment slurries. The slurry is directed into a passageway leading to the grinding chamber in such a way as to be enveloped and atomized by a flow of gaseous drying fluid, e.g., high pressure steam, of at least sonic velocity. The solids become at least partially dried before entering the grinding chamber but with little or no tendency to adhere to the walls of the passageway leading thereto.

United States Patent 1191 Coombe et al. 1451 Oct. 8, 1974 FLUID ENERGYDRYING AND GRINDING 3.223.333 12/1965 Stephanoff 241/5 MILL 3.5593952/1971 Fay 241/5 3.667,l3l 6/1972 Stephanoff 1 34/10 inventors: AnthonyJohn Coombe, wilmmg 3,726,483 4/1973 Kometani et 211.. 241/5 EdwardAnthony Danisavich, 3,726,484 4/1973 Shurr 241/5 Newark; Thomas ThaddeusGniewek, Jr.; Philip John Muhlmichl, both of Wilmington; 'm 'f' LakeRobert Ernest stub", Newark, all Assistant Exammer-DeWalden W. Jones ofDel.

[73] Assignee: E. I. du Pont de Nemours and [57] ABSTRACT Company,Wilmington, Del. [22] Filed; July 31 1973 A fluid energy mill of theconfined vortex type is designed to permit the simultaneous drying andgrinding PP 384,355 of slurries of pulverulent solids, e.g., pigmentslurries. The slurry is directed into a passageway leading to the 52 US.Cl 241/39, 34/10, 241/5 grinding Chamber in Such a Way as to beenveloped 51 Int. Cl. B02c 21/00 and atomized by a flow of gaseousdrying fluid, s [58] Field of Search 241/5, 19, 23, 39; 34/10, highPressure Steam, of at least Sonic velocity The 34/57 solids become atleast partially dried before entering the grinding chamber but withlittle or no tendency to 56] References Cited adhere to the walls of thepassageway leading thereto. UNITED STATES PATENTS 5 Claims, 4 DrawingFigures 2,846,l50 8/1958 Work 241/5 PATENI'LD [JET 81374 SHEET 2 BF 2 1FLUID ENERGY DRYING AND GRINDING MILL BACKGROUND OF THE INVENTION Fluidenergy mills of the confined vortex type are well known and widelyemployed in certain industries such as the pigment, cosmetic and plasticindustries because of their efficiency and economy in the grinding ofpulverulent solids. A number of early designs are described inconsiderable detail in US. Pat. No. 2,032,827.

Most such fluid energy mills are variations on a basic configuration ofa generally circular chamber enclosed by a pair of axial wallsand aperipheral wall, the axial length or height of the chamber beingsubstantially less than the diameter. At the periphery of the mill thereis located at least one inlet for injecting the gaseous grinding fluidwhich furnishes the energy for grinding the solids, along with one ormore feed devices for introducing the pulverulent solids to be ground.Preferably several uniformly spaced apart inlets for gaseous grindingfluid are provided around the circumference of the mill and they areoriented generally tangentially to the chamber. An outlet coaxial to andin direct communication with the grinding chamber is provided fordischarge of the ground solids to a cyclone or bag filter forcollection.

Fluid energy mills of theforegoing type combine both grinding andclassification within a single chamber. As the gaseous grinding fluid isfed tangentially into the periphery of the chamber along with the solidsto be ground a vortex is created whereby the particles are swept alongin a spiral path to be eventually discharged at the central outlet. Byproper selection of operating conditions, such as rate and tangency offluid injection, particles above a specific size can be kept within themill until sufficient attrition occurs, whereas other particles areallowed to pass through.

Heretofore such fluid energy mills have not, however, beensatisfactorily operable except with pulverulent solids which were in arelatively dry condition. Poor grinding and even clogging of passagescould occur if excess liquid were present. This has been a disadvantagein certain areas where the treatment of liquid-laden solids such asslurries would be desirable. For example, in the case of Ti pigmentsproduced by the vapor phase oxidation of TiCl,, the particles as formedare frequently collected as a water slurry. The utilization then of afluid energy milling step, invariably needed to break up particleagglomerates, has meant that it would first be necessary to dry theparticles, e.g., via a costly drying operation.

The desirability of a fluid energy mill permitting drying and grindingfunctions to be carried out simultaneously will be apparent.

SUMMARY OF THE INVENTION The present invention relates to a fluid energymill of the confined vortex type as described above but with a feeddevice that enables it to be used for the simultaneous drying andgrinding of flurries of pulverulent solids. More particularly there isused a slurry feed device formed by walls defining a generallyrectilinear passageway which opens into a peripheral region of thegrinding chamber,

means for supplying gaseous drying fluid of at least sonic velocity tothe annular space to thereby envelop and atomize within the passagewayslurry emerging from the discharge nozzle, and

DETAILED DESCRIPTION OF THE DRAWINGS The invention will be furtherdescribed with reference to the drawings, not to scale and with the samereference characters used to denote identical parts, wherein:

FIG. 1 shows a side elevational view, partly in crosssection, of anapparatus of the invention, the slurry feed device being shown generallyas A,

FIG. 2 is a horizontal view, also partly in crosssection, of theapparatus of FIG. 1 taken normal to the axis at the level of the inletjets,

FIG. 3 illustrates in greater detail the slurry feed device A, the viewbeing a horizontal cross-sectional view, and

FIG. 3a is a cross-sectional view taken across 3a-3a' of FIG. 3.

In FIG. 1 and FIG. 2, l is a header for gaseous grinding fluid andencircles peripheral wall 2 of generally circular grinding chamber 3.Inlets 4, of which only four are shown, interconnect the header and thegrinding chamber. Axial walls 5 and 6 of the chamber may be relativelyparallel but in the preferred embodiment, as shown, come closer to oneanother as the chamber axis is approached. Circular discharge port 7 andexhaust duct 8 are axially located. Fluid from each inlet 4 enters theperipheral wall 2 generally tangentially of the chamber, i.e., at anangle that is tangent to a circle about the center of the chamber whichhas a radius smaller than the radius of the chamber. A multiplicity ofthese inlets for gaseous grinding fluid is advantageously used, twelvebeing convenient for a chamber of 27 inches diameter.

A slurry feed device shown generally as A, and to be described morefully hereinafter in connection with FIG. 3, serves to introduceliquid-laden solids, preferably a solids slurry such as a Ti0 pigmentslurry in water, to a peripheral region of the chamber through elongatedpassageway 10, the latter being aligned nearly tangentially toperipheral wall 2 to facilitate flow of the solids into the chambervortex. Passageway l0 preferably opens into the chamber directly throughperipheral wall 2, as shown, but it may alternatively open into thechamber through upper axial wall 5 in close proximity to peripheral wall2. The cylindrical discharge opening formed by discharge port 7, inconjunction with conical enclosure 9, forms a centrifugal separator intowhich the ground product settles to be collected while the grindingfluid flows out through exhaust duct 8.

Referring now to FIG. 3 and to the details of the slurry feed device Afor introducing and drying the slurry, elongated passageway 10, throughwhich the solids ultimately pass into the grinding chamber, is formed byannular walls 21, 22 which abut at flanges 23, 24, are held together bybolts 25, with an O-ring gasket 26 in position as shown. An extension ofwall 22 forms housing 27 which is adapted to accommodate the entry ofhigh velocity gaseous drying fluid, e.g., high pressure steam suppliedvia pipe 28, which intersects passageway at a right angle.

Communicating with passageway 10 in axial alignment with the relativelywider upstream portion thereof is a slurry supply conduit showngenerally as 29 which is connected at its feed inlet 30 to a source ofthe slurry to be dried and ground. Conduit 29 is maintained in positionagainst housing 27 by means of bolts 33. O-rings 35 are in contact withshim plate 34 to assist in preventing leaks of gaseous drying fluid.

Conduit 29 extends into housing 27 and is convergently tapered at itsforward extremity to form discharge nozzle 36. The latter is centeredwithin the wider portion of passageway 10 leaving a small annular space37 between passageway lip 38 and discharge nozzle 36 for flow of gaseousdrying fluid so as to envelop the stream of slurry emerging from thedischarge nozzle and passing into the narrower portion of passageway 10.

By varying the thickness of shim plate 34 the size of annular space 37is varied.

As shown more clearly in FIG. 3a, slurry conduit 29 is composed of innercylindrical element 41 and outer cylindrical element 42, the two bewngwelded at each end. Hence, essentially fully along the length of conduit29 is channel 39 for flow of heat exchange fluid, e.g., cold water,therethrough. Elongated baffles 43 and 44 are welded to cylinder 41 andextend nearly the entire length of channel 39. Thus the heat exchangefluid will for the most part enter at threaded connection 45, traversethe length of conduit 29 through channel 39, return and finally exit atthreaded connection 46. It is especially desirable that the portion ofconduit 29 which is exposed to the high velocity gaseous drying fluidfrom pipe 28 be able to have its temperature appropriately controlled bythe heat exchange fluid.

DESCRIPTION OF SPECIFIC EMBODIMENTS Insofar as the grinding function isconcerned, the operation of the fluid energy mill of the inventionfollows that of similar devices of the prior art. In this respectreference is made to the aforementioned US. Pat. No. 2,032,827 toAndrews and additionally to US. Pat. No. 3,462,086 to Bertrand et al.,the disclosures of which are incorporated herein by reference.

With respect to the improvement according to the present invention,gaseous drying fluid, preferably steam, flowing through pipe 28 at arelatively low velocity and high pressure, undergoes a marked increasein velocity, to at least sonic velocity, as it emerges from what amountsto a converging-diverging nozzle at annular space 37 and comes incontact with slurry passing through discharge nozzle 36 at a relativelylow velocity. Mass and heat transfer in the region of the dischargenozzle is extremely rapid. Desirably, enough heat will be supplied bythe gaseous drying fluid to permit the temperature of the resultingmixture to remain above saturation temperature. The pulverulent solidswill hence be at least surface dry before entering the grinding chamberwhere they are subjected to the action of a gaseous grinding fluid,which like the gaseous drying fluid is also preferably steam.

The envelope of gaseous drying fluid about the stream of slurry issuingfrom discharge nozzle 36 serves not only in the drying function but alsoin preventing a buildup of solids along the walls of passageway 10.Likewise, a flow of a cooling liquid such as chilled water throughchannel 39 of slurry supply conduit 29 aids in preventing prematuredrying of the slurry on the inner walls of the conduit or on dischargenozzle 36. It is particularly important at start-up that the dischargenozzle temperature not become excessively high before the slurry flow iscommenced in order that solids do not bake out on the inner conduitwall.

While it is preferred to employ a multiplicity of inlets 4 for gaseousgrinding fluid, as shown in FIGS. 1 and 2, it is also practical to omitthem under certain circumstances. For example where the grindingfunction is only of secondary importance as compared to the dryingfunction, the gaseous drying fluid supplied to chamber 3 via passageway10 can be adequate to serve as grinding fluid as well.

While the present invention is particularly described with reference tothe treatment of aqueous TiO slurries, it will be apparent that it isalso applicable to use with various other materials as well.

EXAMPLE The material to be dried and ground is a slurry in water ofuncoated rutile TiO particles having an average particle size of about0.22 micron. The solids content of the slurry is 64% by weight and itsfeed rate is 6100 pounds per hour.

The fluid energy mill is that described in connection with the drawings.The axial-walls converge from a height of 3% inch at the periphery to 2%inch at the discharge port. The grinding chamber is 27 inches indiameter. There is a series of 12 tangential ring jets as inlets forflow of 5l8F., and psig steam at the rate of 3200 pounds per hour intothe grinding chamber.

The drying fluid is 800F. steam fed to a 278 inch internal diametersupply pipe at 450 psig and at the rate of 8000 pounds per hours. Theannular clearance aurrounding the discharge nozzle of the slurry supplyconduit is 0.070 inch. The passageway is 1 /8 inches in diameter at thenarrowest portion just beyond the discharge nozzle; and expands to 2 /8inches in diameter its length from the nozzle to the grinding chamber is19 inches.

The temperature of the steam discharged from the grinding chamber is338F. The overall steam to pigment ratio is 3.0.

The quality of the product is rated equivalent to the same TiO driedseparately and ground in a conventional manner. No pluggage of theslurry feed device is encountered throughout an extended run.

What is claimed is:

1. A fluid energy mill of the confined vortex type for drying andgrinding a slurry of pulverulent solids, said mill comprising:

a generally circular grinding chamber defined by a pair of opposingaxial walls and a peripheral wall,

a slurry feed device formed by walls defining a generally rectilinearpassageway which opens into a peripheral region of said chamber, aslurry supply conduit having an inlet remote from said passageway and adischarge nozzle posiof said slurry feed device opens into theperipheral wall of said grinding chamber.

3. Millaccording to claim 1 wherein the passageway of said slurry feeddevice opens generally tangentially into the peripheral wall of saidgrinding chamber.

4. Mill according to claim 1 wherein the means for controlling thetemperature of the discharge nozzle comprises a channel inside the wallsof the slurry supply conduit for flow of a heat exchange mediumtherethrough.

5. Mill according to claim 1 wherein the discharge nozzle forms aconverging-diverging nozzle with the adjacent portion the passageway.

1. A fluid energy mill of the confined vortex type for drying andgrinding a slurry of pulverulent solids, said mill comprising: agenerally circular grinding chamber defined by a pair of opposing axialwalls and a peripheral wall, a slurry feed device formed by wallsdefining a generally rectilinear passageway which opens into aperipheral region of said chamber, a slurry supply conduit having aninlet remote from said passageway and a discharge nozzle positioned insaid passageway for directing slurry along the passageway and towardsaid chamber, the size of discharge nozzle being small relative to thesurrounding passageway to define an annular space therebetween, meansfor supplying gaseous drying fluid of at least sonic velocity to saidannular space to thereby envelop and atomize within the passagewayslurry emerging from said discharge nozzle, means for controlling thetemperature of said discharge nozzle, and discharge means forwithdrawing pulverulent solids and gaseous grinding fluid along the axisof the chamber.
 2. Mill according to claim 1 wherein the passageway ofsaid slurry feed device opens into the peripheral wall of said grindingchamber.
 3. Mill according to claim 1 wherein the passageway of saidslurry feed device opens generally tangentially into the peripheral wallof said grinding chamber.
 4. Mill according to claim 1 wherein the meansfor controlling the temperature of the discharge nozzle comprises achannel inside the walls of the slurry supply conduit for flow of a heatexchange medium therethrough.
 5. Mill according to claim 1 wherein thedischarge nozzle forms a converging-diverging nozzle with the adjacentportion the passageway.